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I have been watching some videos on cloud chambers:

https://www.youtube.com/watch?v=Efgy1bV2aQo

and they are quite amazing. What I can't figure out, is why aren't these chambers over loaded with tracks when a radioactive source is placed near it? Clearly, they are sensitive enough to pick up all ionization events of a single particle's trajectory. For example, in the above video, the person places Radon-220 in the chamber. By conservative estimates, there should be ~billions of alpha particles emitted. If that is the case, you would never expect an entire particle track to be visible. Either they would be overloaded with condensation, or (if the was a very low probability of condensation) you would see tiny dots all over the place (never an entire track). There are all kinds of other examples where a similar thing occurs (tracks are visible when you might naively expect to see ~ uniform condensation). Does anyone have any thoughts on this? Do cloud chambers only reveal the incredibly rare, super high energy particles, or something? How can they be sensitive enough to record an entire particle's trajectory, but not be overloaded by noise? I.e., how can they only respond to 1 particle/ ~ billion, say?

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  • $\begingroup$ Maybe there is a very low concentration of Radon in the chamber? $\endgroup$ – mmesser314 Nov 4 '14 at 12:42
  • $\begingroup$ It is pretty hard to get lots of Radon 220 when it has a half life under a minute (unless you have a lot of Thorium powder laying around to suck it out of). $\endgroup$ – Jon Custer Nov 4 '14 at 14:19

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